I have one son who was good at math, but didn't particularly ENJOY it. He was also very good at writing, reading, etc. He doesn't think he's that good at math, and my husband agrees that it doesn't come EASILY to him (it's not "intuitive"), but he did get good grades. This was my son who was good at building with Legos, but he always had to use the instructions -- he never creatively tried to build stuff on his own.
My younger son is GREAT at math, depending on his teacher. If he has a great teacher, it all clicks for him. But again, it's not "intuitive". Also, he has a difficult time with reading and writing. This son, like yours, has had very little interest in Lego.
I've read about many people on this forum who aren't necessarily great at math, but are wonderful writers.